1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers. More specifically, it relates to vertical cavity surface emitting lasers having heat spreading layers that assist single mode operation.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. In a VCSEL, optical emission occurs normal to the plane of a PN junction. VCSELs have certain advantages over edge-emitting laser diodes, including smaller optical beam divergence and better-defined and more circular laser beams. Such advantages make VCSELs well suited for optical data storage, data and telecommunication systems, and laser scanning.
VCSELs can be formed from a wide range of material systems to produce specific characteristics. VCSELs typically have active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD).
To assist the understanding of VCSELs, FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAs) substrate 12 is disposed with an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAS substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20 with quantum wells is formed over the lower spacer 18. A p-type graded-index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that is formed by implanting protons into the top mirror stack 24, or by forming an oxide layer. In either event, the insulating region 40 has a conductive annular central opening 42 that forms an electrically conductive path though the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and its conductive central opening 42 confine the current 21 flow through the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, providing a p-type substrate), different material systems can be used, operational details can be varied, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs are not without problems. One set of problems is particularly prevalent in high power data and telecommunication applications. Such applications often require a high power single mode laser light source that operates at a long wavelength, such as 1310 or 1550 nanometers, and that illuminates a single mode optical fiber. Because of their wide range of material systems, VCSELs can operate at such wavelengths. Furthermore, single mode VCSEL operation is also well known. However, efforts to produce suitable high power single mode, long wavelength VCSEL sources have been plagued by temperature performance problems, specifically the ability to meet minimum power requirements while maintaining single transverse mode operation.
Meeting commonly required power output is a particular problem because a VCSEL has an active region with a small volume. Increasing the optical output power increases the active region temperature, which tends to produce multiple transverse optical modes. Alternatively, producing a VCSEL with a highly stable single mode operation typically requires a high lasing threshold current, which tends to produce a low optical output and/or poor electrical response. Furthermore, some highly stable single mode VCSELs have refractive index differences between where current is injected into the active region and peripheral areas. This produces a thermal lens effect and poor optical confinement.
Another problem with incorporating VCSELs into high power data and telecommunication systems is aligning the system's optical fiber with light emitted from a VCSEL. Thermal problems can complicate alignment, particularly when external cooling devices or structures are used to reduce the operating temperature of the VCSEL.
Therefore, a new type of highly stable, single mode VCSEL would be beneficial. Even more beneficial would be a new type of highly stable, high power, single mode VCSEL that is suitable for use in long wavelength applications. Still more beneficial would be a technique of aligning the optical output of a VCSEL to an optical fiber.